When defining assemblies, we should avoid having an over defined assembly. However, there are certain advantages of having our assembly under defined or over defined. Here are some scenarios for both cases:

• If the assembly has a moving part, a common practice will be to have the assembly under defined so that the desired movement is visible if we were to drag and move the part around. For example, if we assemble a windmill, we can choose to have the blades under defined to show how they move and how all the parts interact with each other during that movement.
• If all the parts in the assembly are fixed, a common practice will be to have the assembly fully defined. A common example of fully fixed assemblies are tables, which don't have any moving parts. 

As we can see, keeping our assemblies fully defined or under defined has certain advantages. As designers or draftsmen, we will have to weigh up the advantages of each and adapt our own approach. Next, we will learn how to view and adjust active mates.